Semiconductor mechanical sensor

ABSTRACT

A semiconductor mechanical sensor having a new structure in which a S/N ratio is improved. In the central portion of a silicon substrate  1 , a recess portion  2  is formed which includes a beam structure. A weight is formed at the tip of the beam, and in the bottom surface of the weight in the bottom surface of the recess portion  2  facing the same, an electrode  5  is formed. An alternating current electric power is applied between the weight portion  4  and the electrode  5  so that static electricity is created and the weight is excited by the static electricity. In an axial direction which is perpendicular to the direction of the excitation of the weight, an electrode  6  is disposed to face one surface of the weight and a wall surface of the substrate which faces the same. A change in a capacitance between the facing electrodes is electrically detected, and therefore, a change in a physical force acting in the same direction is detected.

This is a division of application Ser. No. 09/181,615, filed Oct. 28,1998 which was a division of application Ser. No. 08/834,129 filed Apr.14, 1997, now U.S. Pat. No. 5,872,024 (per PALM) which was a division ofapplication Ser. No. 08/508,170 filed Jul. 27, 1995, now U.S. Pat. No.5,627,318; which was a division of application Ser. No. 08/109,504 filedAug. 20, 1993, now U.S. Pat. No. 5,461,916.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor mechanical sensor amethod of manufacturing a same, and more particularly, to a method formanufacturing a acceleration sensor or a yaw rate sensor.

2. Description of the Related Art

As a semiconductor mechanical sensor such as an acceleration sensor, ayaw rate sensor, or sensors using piezoelectric ceramics are in wide usefor attitude control of an automobile and to prevent jitter in acommercial video camera. In addition, Japanese Patent PublicationGazette No. 3-74926 discloses that two piezoelectric resistor elementsarranged in parallel to a longitudinal axis of the cantilever, and in aside-by-side configuration, detects a force which corresponds to arotation speed. In other words, without detecting deformation due tovibration of the cantilever, only deformation due to twisting of thecantilever is detected by the piezoelectric resistor element.

However, regarding accuracy, cost, etc., existing yaw rate sensors arenot satisfactory, which restricts their application to other purposes.

SUMMARY OF THE INVENTION

It is an object of the present invention to solve such a problem and tooffer a semiconductor mechanical sensor having a new structure.

A further object of the present invention is to provide a method ofmanufacturing a sensor to improve the S/N ratio in such a semiconductormechanical sensor having a new structure.

A still further object of the present invention is to offer asemiconductor mechanical sensor using a beam deflection type capacitydetection method and a method of manufacturing the same, and to offer asemiconductor mechanical sensor which can detect mechanical changes intwo or three directions (when two such semiconductor mechanical sensorsare used) and a method of manufacturing the same.

To achieve these objects, basically, a semiconductor mechanical sensoraccording to the present invention has a structure as follows. That is,the semiconductor mechanical sensor manufactured according to the methodof the present invention comprises:

a semiconductor substrate;

a beam which is formed on the semiconductor substrate, the beam having aweight; a first pair of electrodes one of which is formed on a firstsurface of the weight and another one of which is formed on a firstsurface of a wall of the substrate opposite to the same surface of theweight; and a second pair of electrodes which arranged perpendicular tothe first pair of electrodes and one of which is formed on a secondsurface of the weight different from the first surface thereof andanother one of which is formed on a second surface of a wall differentfrom the first surface of the wall of the substrate, and opposite to thesame surface of the weight.

In other aspect of the present invention, in addition to the abovestructure, the semiconductor mechanical sensor comprises: an AMmodulation circuit for superimposing a signal from the physical forcedetect electrode onto a carrier wave; and a band pass filter for passinga signal from the AM modulation circuit whose center frequency coincideswith the carrier wave.

In a further aspect of the present invention, a method of manufacturingsuch a semiconductor mechanical sensor comprises the steps of:

a first step of forming a groove of a predetermined depth in a mainsurface of a monocrystalline silicon substrate and perpendicular to themain surface thereof, to thereby form a beam which has a weight;

a second step of forming a pair of electrodes which face each other, oneof which is provided on a side surface of the weight formed in a surfacelayer of the substrate and another one of which is provided on an innersurface of the groove opposite to the side surface of the weight, andforming another electrode on a surface of the weight in a directionwhich is perpendicular to the groove;

a third step of filling the groove with a filling material, forming anelectrode on a bottom surface of the groove and opposite to the otherelectrode which is formed on the surface of the weight with the fillingmaterial interposed therebetween to thereby form another pair ofelectrodes, and of smoothing the major surface of the monocrystallinesilicon substrate;

a forth step of combining the main surface of the monocrystallinesilicon substrate with a separately prepared substrate;

a fifth step of polishing a back surface of the monocrystalline siliconsubstrate to remove a predetermined amount thereof to thereby make themonocrystalline silicon substrate thin; and

a sixth step of etching the filling material in the groove in themonocrystalline silicon substrate to thereby form the beam which has theweight.

In other words, in the semiconductor mechanical sensor manufacturedaccording to the method of present invention, the weight which is formedat the tip of the beam is excited due to static electricity which iscreated by applying an alternating current electric power to a side wallof the substrate which faces one surface of the weight. In such a state,in the axial direction which is perpendicular to the excitationdirection of the weight, a change in the capacitance value between twoelectrodes arranged oppositely to each other is electrically detected sothat a mechanical force which acts and changes in the same directionsuch as a yaw rate, an acceleration or the like is detected.

More precisely, in the semiconductor mechanical sensor according to thepresent invention, the weight is excited by static electricity due toalternating current electric power, and in the axial direction which isperpendicular to the direction of the excitation, a change in thecapacitance value between the two electrodes arranged oppositely to eachother, is electrically detected. The detected signal is superimposed onthe carrier wave in the AM modulation circuit so that the carrier waveis AM modulated. Further, the signal from the AM modulation circuit ispassed through the band pass filter which has a center frequency whichcoincides with the frequency of the carrier wave.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view of a semiconductor mechanical sensor;

FIG. 2 is a view showing a cross section of FIG. 1 taken along the lineA—A;

FIG. 3 is a view showing an electric circuit of a semiconductormechanical sensor;

FIG. 4 is a view showing the waveform of an input signal;

FIG. 5 is a view showing a quantity of displacement;

FIG. 6 is a view showing a signal waveform;

FIG. 7 is a view showing a signal waveform;

FIG. 8 is a view showing a quantity of displacement;

FIG. 9 is a view showing a signal waveform;

FIG. 10 is a plan view of a semiconductor mechanical sensor according toanother embodiment.

FIG. 11 is a view showing a cross section of FIG. 10 taken along theline B—B;

FIG. 12 is an explanatory diagram showing the principles of the presentinvention;

FIG. 13 is a view showing an electric circuit of a semiconductormechanical sensor;

FIG. 14 is a plan view of a semiconductor mechanical sensor;

FIG. 15 is a view showing a cross section of FIG. 14 taken along theline A—A;

FIG. 16 is a cross-sectional view of a semiconductor mechanical sensoraccording to another embodiment of the present invention;

FIG. 17 is a schematic plan view of the semiconductor mechanical sensoraccording to the embodiment shown in FIG. 16;

FIG. 18 is a plan view of the semiconductor mechanical sensor accordingto the embodiment shown in FIG. 16;

FIGS. 19 to 31 are cross-sectional views each showing a configuration ofan intermediate material in respective manufacturing steps;

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following description, semiconductor mechanical sensors accordingto embodiments of the present invention will be described with referenceto the drawings.

FIG. 1 is a plan view of a semiconductor mechanical sensor according toan embodiment of the present invention and FIG. 2 is a view showing across section of FIG. 1 taken along the line A—A. In the descriptionhereinafter, to explain three dimensional directions, a right-to-leftdirection will be referred to as the X-axis direction, an up-downdirection will be referred to as the Y-axis direction and a directionwhich is perpendicular to the drawing sheets will be referred to as theZ-axis direction.

FIG. 1 is a plan view showing a basic structure of a semiconductormechanical sensor according to the present invention. The semiconductormechanical sensor comprises: a semiconductor substrate 1; a beam 3 whichis formed on the semiconductor substrate 1, the beam having a weight 4;a first pair of electrodes 5 which is formed on one surface of theweight 4 and a wall surface which corresponds to the weight surface; anda second pair of electrodes 6 which is formed on one surface of theweight 4 and a wall surface which corresponds to the weight surface inan axial direction of the weight 4 which is perpendicular to the firstpair of electrodes 5.

More particularly, as clearly shown in FIGS. 1 and 2, the siliconsubstrate 1 is a flat plate having a rectangular shape. In the centralportion of the silicon substrate 1, a rectangular recess portion 2 isformed (depth; T). Within the recess portion 2, the beam 3 which has anarrow width (width; W_(B)) extends from the left wall of the recess. Atthe tip of the beam 3, the weight portion 4 is formed with a widthgreater than the beam 3 and a square shape. The beam 3 and the weightportion 4 have the same thickness. Further, one side surface of theweight portion 4 (the top surface in FIG. 1) and the inner wall of therecess portion 2 are spaced away from each other by a small distance(distance d1). In a similar manner, the other side surface of the weightportion 4 (the bottom surface in FIG. 1) and the inner wall of therecess portion 2 are spaced away from each other by the same smalldistance (distance d1). Similarly, the bottom surface of the weightportion 4 and the beam 3 (the bottom surface in FIG. 2) and the bottomsurface of the recess portion 2 are spaced away from each other by asmall distance (distance d2).

Thus, the sensor has a cantilever structure. In this structure, thespace having the distance d2 is created by etching a layer which ispredeterminedly designed to be removed by a surface micro machiningtechnique.

In addition, the beam 3 forms a wiring region for the weight portion 4which serves as an electrode.

In the bottom surface of the recess portion 2, at a region where therecess portion 2 faces the weight portion 4, the electrode portion 5 isformed, and a portion which faces the electrode portion 5, i.e., theweight portion 4 serves as an electrode. Further, the electrode portion6 is formed in a surface of the inner wall of the recess portion 2facing a side of the weight portion 4 (i.e., the upper surface of therecess portion 2 in FIG. 1), and a portion facing the electrode portion6, i.e., the weight portion 4 serves as an electrode. The electrode 5 isan electrode which provides static electricity. The electrode 6 is anelectrode which detects a displacement of the weight portion 4 and formsa capacitance with the weight portion 4. In this structure, the weightportion 4 and the electrodes 5 and 6 are insulated from each other.

FIG. 3 is a view showing an electrical circuit which is used in thesemiconductor mechanical sensor according to the present invention.

That is, as shown in FIG. 3, a circuit for effectively operating thesemiconductor mechanical sensor according to the present inventioncomprises: oscillation means 8 which is connected to a capacitor portion7 which is formed by an electrode 4′ which is disposed on a side wallportion of the weight portion 4 and an electrode 6 which is disposed ona wall surface of the substrate facing the weight portion 4; impedancematching means 12 which is connected to the capacitor portion 7;inverting amplifier means 13 which is connected to the impedancematching means 12; clock signal generation means 17; and sample-and-holdmeans 26 which is connected to the inverting amplifier means 13 andclock signal generation means 17. In response to sample-and-hold periodswhich are determined based on a clock signal which is output by theclock signal generation means 17, the sample-and-hold means 26 records apeak output value of the inversion amplifier means during eachsample-and-hold period and calculates a difference between the peakvalues in different sample-and-hold periods. Differential amplifiermeans 35 is provided for amplifying the difference value.

In other words, in the electrical circuit which is used in the presentinvention, the capacitor portion 7 is formed by the electrode 6 and theweight portion 4, and the oscillator 8 is connected to the weightportion 4 side of the capacitor portion 7. An impedance Z_(L) is formedby a capacitor 9 and a resistor 10 connected to the electrode 6 side ofthe capacitor portion 7. A power source 11 is connected to the capacitor9.

To one end of the impedance ZL, the impedance matching means 12,comprising an operational amplifier, is connected at a point a which iscreated by a change in the capacitance value of the capacitor portion 7.Here, an alternating current voltage source V_(S) (=V·sin ω_(S)t) shownin FIG. 4 is applied between the electrode 5 and the weight portion(electrode) 4 of FIG. 2. In such a state, when the weight portion 4 isdisplaced by Coriolis deflection as shown in FIG. 5, a waveform as shownin FIG. 6 appears at a non-inverted input terminal of the impedancematching means 12 (the point a in FIG. 3).

The output of the impedance matching means 12 of FIG. 3 is coupled tothe inverting amplifier circuit 13. The inverting amplifier means 13 isformed by an operational amplifier 14 and resistors 15 and 16. A signalfrom the impedance matching means 12 is inverted and amplified by theinverting amplifier means 13.

The clock signal generation means 17 is comprised of a voltage adjustor18, two comparators 19 and 20, power sources 21 and 22, a NOR gate 23, aresistor 24 and a capacitor 25. In the clock signal generation means 17,sample-and-hold periods T1 and T2 shown in FIG. 6 are generated.

The sample-and-hold circuit 26 is formed by two operational amplifiers27 and 28, switches 29, 30, 31 and 32 and capacitors 33, 34, 47 and 48.In the sample-and-hold periods T1 and T2 shown in FIG. 6 generated bythe clock signal generation means 17, the switches 29, 30, 31 and 32 areopened and closed, whereby a sample-and-hold operation is performedduring these periods.

The differential amplifier circuit 35 is formed by operationalamplifiers 36, 37 and 38, resistors 39, 40, 41, 42, 43, 44 and 45 and apower source 46. From an output value available from the sample-and-holdcircuit 26, a difference between peak values during the sample-and-holdperiods T1 and T2 is calculated (i.e., A in FIG. 6) and amplified.

At the output terminal of the operational amplifier 38, a sensor outputV_(out) is obtained.

Next, functions of a semiconductor mechanical sensor having aconstruction as explained above will be described with reference to FIG.12.

As shown in FIG. 12, in the present invention, the beam structure 3 isformed in a portion of the semiconductor substrate 1 spaced away fromthe semiconductor substrate 1, and an alternating current electric poweris applied to a wall surface of the substrate which faces one surface ofthe weight 4 which is formed at the tip of the beam 3, so as to generatestatic electricity and excite the weight. In the axial direction whichis perpendicular to the direction of the excitation of the weight, theelectrodes are disposed in a facing relation with each other on the wallsurfaces of the substrate which face the one surface of the weight andthe surface of the beam. A change in the capacitance value between thefacing electrodes is electrically detected so that a mechanical forceswhich act thereto in the same direction is detected.

Between the electrode 5 and the weight portion (electrode) 4 of FIG. 12,an alternating current voltage V_(S) (=V·sin ω_(S)t) is applied whereω_(S) is a rotational angular velocity. As a result, static electricpower F_(E) as defined by Equation 1 below is created.

F_(E)=∈₀·S·V_(S) ²/2d²  (1)

In the direction Z, a displacement as defined by Equation 2 below isgenerated. $\begin{matrix}{D_{Z} = {\frac{F_{E} \cdot L^{3}}{3 \cdot E \cdot I_{Z}} + \frac{F_{E} \cdot L^{2} \cdot L_{m}}{2 \cdot E \cdot I_{Z}}}} & (2)\end{matrix}$

where ∈₀ is a dielectric constant, S is a facing area of the electrodes,d is a distance between the electrodes, L is the length of the beam,L_(m) is the length of the weight portion 4, I_(Z) is a secondary momentof area of the beam 3 in the Z-axis direction, and E is a Young'smodulus.

Differentiating Equation 2 by time t, the velocity V_(Z) vibrates as:

V_(Z) =dD_(Z)/dt  (3)

At this stage, with a rotational angular velocity ω applied to the axisX which is perpendicular to the axis Z, the Coriolis effect Fc definedby

Fc=2mV_(Z)ω  (4)

is created in the axis-Y direction.

In Equation 4, m is the mass of the weight portion 4.

Due to the Coriolis effect Fc, a displacement D_(Y) which is expressedby Equation 5 below is generated in the Y-axis direction.$\begin{matrix}{D_{Y} = {\frac{F_{C} \cdot L^{3}}{3 \cdot E \cdot {IY}} + \frac{F_{C} \cdot L^{2} \cdot L_{m}}{2 \cdot E \cdot {IY}}}} & (5)\end{matrix}$

where IY is a secondary moment of area in the axis-Z direction. Hence, acapacitance between the electrodes C_(Y) is expressed by Equation 6below. $\begin{matrix}{C_{Y} = {ɛ_{0}\frac{S_{y}}{{dy} + {DY}}}} & (6)\end{matrix}$

where S_(y) is the faced area of the electrodes and d_(y) is thedistance between the electrodes.

Due to a change in the value C_(y), a voltage Vω defined by Equation 7is created at the output terminal (output voltage) V_(out).$\begin{matrix}{{V\quad \omega} = {\frac{Z}{Z + {{1/\omega_{s}}C_{y}}} \cdot V_{S}}} & (7)\end{matrix}$

In other words, the output Vω changes in accordance with the rotationalangular velocity ω and the angular velocity ω is calculated as thechange in the value Vω.

Next, a description will be given of how the signal is processed in thecircuit with reference to FIG. 3.

The input waveform applied to the weight portion 4 is a sinusoidal waveas shown in FIG. 4. Because of the Coriolis effect, the weight portion 4is displaced in accordance with a sinusoidal wave which has a frequencydouble that of the input signal as can be seen from Eq. 5 and FIG. 5.This creates a waveform at the non-inverted input terminal α of theimpedance matching means 12 of FIG. 3, as shown in FIG. 6.

The most largely deformed portions of the input waveform of thecapacitor portion 7 during the sample-and-hold periods T1 and T2 shownin FIG. 6, i.e., the portions corresponding to the peak displacement ofthe weight portion 4 are peak-held by the operational amplifiers 27 and28, and the resultant difference is amplified by the operationalamplifiers 36 and 37, whereby the voltage output V_(out) whichcorresponds to the angular velocity ω is calculated.

Next, we assume that an acceleration of a frequency fa (in the directionY) is applied as a disturbance noise. Here, if the relation

fa<<2πω_(S)  (8)

holds, with respect to the input waveform shown in FIG. 7, theacceleration is regarded as a displacement only on one side as shown inFIG. 8, and therefore, the output waveform shown in FIG. 9 does notinclude a deformed portion.

In the processing in the circuit shown in FIG. 3, this waveform iscancelled. For instance, where the characteristic frequency of thecantilever 3 is 4 KHz and ω_(S)/2π=3 KHz, since the frequency componentof acceleration of an automobile is around 300 Hz at maximum, Eq. 8holds.

Further, since the frequency component is even smaller for displacementdue to temperature, Eq. 8 holds satisfactorily.

In this manner, in a sensing operation, the processing circuit cancelsmost noises interfering with detection or the angular velocity. Hence,the angular velocity is detected accurately.

In addition, in the electrical circuit as above according to the presentinvention, since a deformed waveform of the beam due to acceleration anda deformed waveform of the beam due to a yaw rate are different fromeach other and clearly distinguishable from each other, thesemiconductor mechanical sensor according to the present invention canbe used as both an acceleration sensor and a yaw rate sensor, as well asfor other sensors.

As described above, in the above example of the present invention, thebeam structure is formed in a portion of the silicon substrate 1(semiconductor substrate) spaced away from the silicon substrate 1, andan alternating current electric power is applied to a wall surface ofthe substrate which faces one surface of the weight which is formed atthe tip of the beam, so as to deflect the weight by static electricity.In the axial direction perpendicular to the direction of the excitationof the weight, the electrodes 6 are disposed in a facing relation on thewall surfaces of the substrate facing the one surface of the weight andthe surface of the beam. A change in the capacitance value between thefacing electrodes is electrically detected so that mechanical forceswhich act in the same direction, i.e., an acceleration or a yaw rate, isdetected. Thus, the semiconductor mechanical sensor has a new structure.

The present invention is not limited to the example above. For example,as shown in FIGS. 10 and 11, as a portion to which static electricity isto be applied, an excitation electrode 48 may be disposed in one sidewall of the recess portion 2, and a detect electrode 49 may be disposedon the bottom surface of the recess portion 2.

As hereinabove described in detail, the present invention provides asemiconductor mechanical sensor which has a new structure.

Incidentally, the semiconductor mechanical sensor structure as above hasan inconvenience that in amplifying a signal of the sensing part, noise(e.g., thermal noise, 1/f noise) is also amplified, which makes itdifficult to improve the S/N ratio.

As a result of study devoted to solving this problem, the inventor ofthe present invention has come to the conclusion that the problem can besolved if the semiconductor mechanical sensor described above furthercomprises an AM modulation circuit for superimposing a signal from thephysical force detecting electrode onto a carrier wave, and a band passfilter for passing a signal from the AM modulation circuit whose centerfrequency coincides with the carrier wave.

In the following, an embodiment of a circuit structure of the exampleabove according to the present invention will be described withreference to the drawings. FIG. 13 is a plan view of an electricalcircuit according to the present invention, FIG. 14 is a plan viewshowing a semiconductor mechanical sensor, and FIG. 15 is a view showinga cross section of FIG. 14 taken along the line A—A. In the descriptionbelow, to explain three dimensional directions, a right-to-leftdirection will be referred to as the X-axis direction, an up-downdirection will be referred to as the Y-axis direction and a directionwhich is perpendicular to the drawing sheets will be referred to as theZ-axis direction.

FIG. 14 shows an example where a semiconductor mechanical sensor devicecomprises two semiconductor mechanical sensors according to the presentinvention disposed as a pair. In such a structure, a change in a certainphysical force and a change in a different physical force can beseparately detected and detection of a change in the physical force canbe achieved accurately, for instance.

In FIG. 14, a silicon substrate 51 is a flat plate and includes arectangular recess portion 52 (depth; T). Within the recess portion 52,two beams 53 extend from the left side of FIG. 14. At the tips of thebeams 53, a weight 55 is formed. On the other hand, within the recessportion 52, two beams 54 extend from the right side of FIG. 14, and atthe tips of the beams 54, a weight 56 is formed. The weights 55 and 56are wider than the beams 53 and 54 and each is shaped in a rectangularshape. The beams 53 and 54 and the weights 55 and 56 have the samethickness.

In addition, one side surface of the weight 55 (the top surface in FIG.14) and the inner wall of the recess portion 52 are spaced away fromeach other by a small distance (distance a). In a similar manner, theother side surface of the weight 55 (the bottom surface in FIG. 14) andthe inner wall of the recess portion 52 are spaced away from each otherby the same small distance a. Similarly, the bottom surface of theweight 55 (the bottom surface in FIG. 15) and the bottom surface of therecess portion 52 are spaced away from each other by a small distance(distance d1).

On the other hand, one side surface of the weight 56 (the top surface inFIG. 14) and the inner wall of the recess portion 52 are spaced awayfrom each other by the same small distance. In a similar manner, theother side surface of the weight 56 (the bottom surface in FIG. 14) andthe inner wall of the recess portion 52 are spaced away from each otherby the same small distance a. Similarly, the bottom surface of theweight 56 (the bottom surface in FIG. 15) and the bottom surface of therecess portion 52 are spaced away from each other by the small distanced1.

Thus, the illustrated sensor has a cantilever structure. In thisstructure, the distance d1 is created by etching a layer which ispredeterminedly designed to be removed, by a surface micro machiningtechnique.

In FIG. 15, in the bottom surface of the recess portion 52 where therecess portion 52 faces the weights 55 and 56, electrodes 57 and 58 areformed. In portions of the weights 55 and 56 where they face theelectrodes 57 and 58, electrodes 59 and 60 are formed. Further, in aninner wall surface of the recess portion 52 where the recess portion 52faces the weights 55 and 56 (i.e., in the upper surface of the recessportion 52 in FIG. 14), electrodes 159 and 160 are formed, and inportions of the weights 55 and 56 where they face the electrodes 159 and160, electrodes 61 and 62 are formed.

In an inner wall surface of the recess portion 52 where the recessportion 52 faces the weights 55 and 56 (i.e., in the lower surface ofthe recess portion 52 in FIG. 14), electrodes 63 and 64 are formed, andin portions of the weights 55 and 56 where they face the electrodes 63and 64, electrodes 65 and 66 are formed.

In addition, in this structure, the electrodes 57, 58, 59, 60, 61, 62,63, 64, 159 and 160 are insulated from each other.

A capacitor C_(s+) is created by the electrodes 59 and 57, a capacitorC_(s−) is created by the electrodes 60 and 58, a capacitor C_(d+) iscreated by the electrodes 159 and 61, a capacitor C_(d−) is created bythe electrodes 64 and 66, a capacitor C_(e+) is created by theelectrodes 65 and 63, and a capacitor C_(e−) is created by theelectrodes 160 and 62.

The beams 53 and 54 form wiring regions for the electrodes 59 (61, 65)and 60 (62, 66), respectively.

For clarity of explanation, although the electrodes 59, 61 and 65 aredescribed as different electrodes, they are one and the same electrode(same potential). Likewise, although described as different electrodesfor clarity of explanation, the electrodes 60, 62 and 66 are one and thesame electrodes (same potential).

FIG. 13 shows an electrical circuit of the semiconductor mechanicalsensor according to the present invention.

The processing circuit of the sensor comprises an oscillator 67, asensing part 68, a differential amplifier 69, a band pass filter 70, asample-and-hold circuit 71 and a subsequent stage amplifier 72.

A capacitor Cr of FIG. 13 is not shown in FIGS. 14 and 15. However, thecapacitor Cr is connected in parallel with a resistor R and has a fixedcapacitance value Cr=C_(s+)=C_(s−).

The capacitors C_(e+) and C_(e−) drive the weights 55 and 56 by staticelectric force Fe. The capacitors C_(s+) and C_(s) are capacitors fordetecting the amount of displacement of the weights 55 and 56 in theZ-axis direction due to the Coriolis effect Fc.

The capacitors C_(d+) and C_(d−) shown in FIG. 14 are monitor capacitorsfor detecting the amount of movement of the weights 55 and 56 in theY-axis direction due to the drive capacitors C_(e+) and C_(e−).

Next, the structure shown in FIG. 13, except for the sensing part 68,will be described.

The oscillator 67 has an oscillation frequency of 10 KHz and provides avoltage (alternating current electric power) for driving the weights 55and 56 and a signal (carrier wave) to the capacitors C_(s+) and C_(s−).The resistor R applies a bias voltage to any one of connection portionsbetween the capacitors C_(s−) or C_(s+) and Crs, and has a resistanceR>>1/ωCr. By applying a bias, each one of the resistors R makessubsequent signal processing possible.

The differential amplifier 69 amplifies a difference voltage betweeninputs (capacitors C_(s+) and C_(s−)). The band pass filter 70 has acenter frequency of 10 KHz which coincides with the frequency of thecarrier wave. In addition, the band pass filter 70 attenuates signalsother than those having a predetermined frequency band (near the centerfrequency). In this example, the band pass filter 70 is formed by aswitched-capacitor filter (S.C.F.).

The sample-and-hold circuit 71 (detector circuit) demodulates a signalwhich is AM modulated as will be described later. An operationalamplifier 73 and resistors 74 and 75 form a reference voltage for usewithin the processing circuit. The subsequent stage amplifier 72amplifies a detected signal. The subsequent stage amplifier 72 may beomitted.

In this example, the electrodes 57, 58, 59 and 60 form a yaw ratedetecting electrode while the oscillator 67 and the differentialamplifier 69 form an AM modulation circuit.

Next, the functions of a semiconductor mechanical sensor having theconstruction described above will be described.

When the oscillator 67 applies a voltage V_(IN) (=V_(CM)·cos ω_(c)t) tothe capacitors C_(e−) and C_(s+), static electric force Fe as defined byEquation 9 below is created.

Fe=(∈₀S/2a²)·V_(IN) ²  (9)

where ∈₀; a dielectric constant

a; a distance between the capacitors C_(e−) and C_(e+)

S; a faced electrode area of the capacitors C_(e−) and C_(e+)

Due to the static electric force Fe, the weights 55 and 56 are displacedin the Y-axis direction. Assuming that the amounts of the displacementsare Dy, the relationship shown in Equation 10 is created.

Dy=KFe  (10)

where K: a constant which is determined by the cantilever. Here, it isto be noted that the weights 55 and 56 move in different directions.

From Eqs. 9 and 10, where the velocities in the Y-axis direction of theweights 55 and 56 are V_(y55) and V_(y56), respectively, the followingequation (11) is obtained. $\begin{matrix}\begin{matrix}{V_{y55} = {- V_{y56}}} \\{= {{K \cdot \left( {ɛ_{0}{S/4}a^{2}} \right) \cdot V_{CM}^{2} \cdot 2}{\omega_{c} \cdot \sin}\quad 2\omega_{c}^{t}}}\end{matrix} & (11)\end{matrix}$

At this stage, if the axis X is the rotation axis, and when the weightis rotated with respect to the axis X rotates at the angular velocity ω,Coriolis effect F_(c55)=2mV_(y55)ω, F_(c56)=2mV_(y56)ω are created atthe axis z.

As a result, the weights 55 and 56 are displaced in the Z-axisdirection. Assuming that the displacements are D_(z55) and D_(z56),

D_(z55)=L_(55·F) _(c55)

D_(z56)=L_(56·F) _(c56)  (12)

where L₅₅, L₅₆ are constants which are determined by the cantilever.

If the weights 55 and 56 and the cantilever are formed to have the samedimensions, L₅₅=L₅₆, and hence, |D_(z55)|=|D_(z56)|=Δd.

In other words, the capacitance values of C_(s+) and C_(s−) are

C_(s+)=(∈₀·S)/(d+Δd)

C_(s−)=(∈₀·S)/(d−Δd)  (13)

Hence, an output V_(pre) of the differential amplifier 69 is

V_(pre)=V_(IN)·{C_(S+)/(C_(S+)+Cr)−C_(S−)/(C_(S−)+Cr)}·AV1≈V_(IN)·(−Δd/2d)·AV1  (14)

where VA1 is an amplification factor of the differential amplifier 67.

From Eqs. 11 and 12, Δd is

Δd=L₅₅·2m·K(∈₀·S/4a^(2)·V) _(CM) ²·2ω_(c)·ω·sin 2ω_(c)t  (15)

On the other hand, from Eqs. 14 and 15,

V_(pre)=AV1·V_(CM) ³·L₅₅·2m·K(∈₀·S/4a²)·ω_(c)·ω·(sin ω_(c)t+sin3ω_(c)t)  (16)

In Eq. 16, VCM3·L₅₅·2m·K(∈0·S/4a2)·ω_(c) on the right side is a constantwhich is determined by the structure of the cantilever and a conditionof the input voltage. From Eq. 16, it is understood that the valueV_(pre) indicates a voltage which is in proportion to the angularvelocity ω which is to be detected. The value V_(pre) is expressed as avoltage output which is AM modulated to the frequency of the inputsignal f_(IN)=ω_(c)/2π and a frequency which is triple the same.

The foregoing has referred to a detected signal alone. However, noisemay be generated by circuit elements of the differential amplifier 69when a signal is processed in the differential amplifier 69, and noisemay be introduced into the power source system from outside. Thesenoises are also amplified by the differential amplifier 69. Hence, fromEq. 16,

V_(pre) =AV1·V_(CM) ³·L₅₅·2m·K(∈₀·S/4a²)·ω_(c)·ω·(sin ω_(c)t+sin3ω_(c)t)+AV1·V_(N)  (17)

Thus, AV1·V_(N) is created which expresses a noise which degrades theS/N ratio of the angular velocity ω to be detected.

To deal with this, as shown in Eq. 17, signal data concerning theangular velocity to be detected, is AM modulated by a certain modulatorand passed through the band pass filter 70, having a center frequencyf_(c)=ω2π, whereby the S/N ratio is improved.

Assume that an output of the band pass filter 70 having 5_(c)=ω_(c)/2πis V_(BPF),

V_(BPF)=AV1·V_(CM) ³·L₅₅·2m·K(Å₀·S/4a²)·ω_(c)·ω·sinω_(c)t+AV1·V_(N)(f_(c))  (18)

The value V_(BPF) is expressed as shown in Fq. 18, and therefore, onlyAV1·V_(N)(f_(c)), i.e., an noise component whose frequency component isequal to f_(c) is left. Hence,

AV1·V_(N)>>AV1·V_(N)(f_(c))  (19)

Thus, an output which is in proportion to the angular velocity ω andwhich has a high S/N ratio is obtained. By processing this output in thesample-and-hold circuit 71 (detector circuit) if necessary, an outputV_(out) which is in proportion to the angular velocity ω is obtained asbelow.

V_(out)≈AV1·V_(CM) ³·L₅₅2m·K(∈₀·S/4a²)·ω_(c)ω  (20)

This output is amplified, if necessary, in the subsequent stageamplifier 72.

As described above, in the present embodiment, the oscillator 67 and thedifferential amplifier 69 (AM modulation circuit) superimpose signalsfrom the electrodes 57, 59 and 58, 60 (yaw rate detect electrodes) on acarrier wave, and a signal from the differential amplifier 69 is passedthrough the band pass filter 70 which has a center frequency whichcoincides with that of the carrier wave. Hence, in processing a signalby the differential amplifier 69, even if noise is generated in acircuit element of the differential amplifier 69 when a signal isprocessed in the differential amplifier 69 and other noise is introducedinto the power source system from outside, these noises are removed.That is, noise (e.g., a thermal noises, a 1/f noise) is deenphasized andtherefore the S/N ratio is improved.

As described above, the present embodiment provides an improved S/Nratio.

However, with respect to a semiconductor mechanical sensor such as thesemiconductor yaw rate sensor above which is movable in two directions,the example described above is insufficient in terms of structure. Tomanufacture the sensor, an efficient manufacturing method for a highproductivity has not been proposed yet.

To deal with this, in addition to the examples described above, thepresent invention offers a semiconductor mechanical sensor which has anoptimum structure and methods of efficiently manufacturing thesemiconductor mechanical sensors according to the examples describedabove. That is, according to an other example of the present invention,a semiconductor mechanical sensor comprises: a thin monocrystallinesilicon substrate which is joined onto a substrate through an insulationfilm; a beam which is formed in the monocrystalline silicon substrateand which has a weight; a first electrode which is formed in one surfaceof said weight and a wall surface which corresponds to said weightsurface; and a second electrode which is formed in one surface of theweight and a wall surface which corresponds to the weight surface in anaxial direction of the weight which is perpendicular to the electrode,and either one of the electrodes is preferably formed on the majorsurface of the monocrystalline silicon substrate in parallel with themonocrystalline silicon substrate.

Further, all electrode contacting portions are preferably formed on thesame surface of the thin monocrystalline silicon substrate.

Describing the semiconductor mechanical sensor according to the presentinvention in more detail, the semiconductor mechanical sensor has astructure in which a plurality of groove portions 201 are formed in thetip portion 139 of a weight portion 139, an electrode is disposed on aninner wall portion of each of groove portions 201, and a fixed member202 extends in each groove portion 201 and an other electrode isdisposed on a side surface portion which faces the inner wall portion ofthe groove portion of the weight portion 4 of the fixed member 202.

In this example, a first electrode and a second electrode which isdisposed in an axial direction perpendicular to th first electrodedetect a mechanical quantity which is applied to a beam having a weight.

Now, a semiconductor mechanical sensor having such a structure accordingto the present invention will be described with reference to FIGS. 16 to18.

FIG. 17 is a schematic plan view of the semiconductor mechanical sensoraccording to the present example. That is, in the illustrated sensor, acantilever 102 is formed in a monocrystalline silicon substrate 101 soas to include a weight 139 at the tip. In a tip portion 200 of theweight 139, three projections 103, 104 and 105 are formed spaced fromeach other to extend along the elongation of the beam, and a grooveportion 201 is formed between the three projections 103, 104 and 105. Onthe monocrystalline silicon substrate 101 side facing the tip portionsurface 200 of the cantilever 102 (weight 139), between the projections103 and 104, two projections 106 and 107 are formed spaced from eachother to extend in parallel with the projections 103 and 104, therebyforming a fixed portion 202. In a similar manner, on the monocrystallinesilicon substrate 101 side facing the tip portion surface of thecantilever 102 (weight 139), between the projections 104 and 105, twoprojections 108 and 109 are formed spaced from each other to extendparallel to the projections 104 and 105.

FIG. 18 is a plan view showing the semiconductor mechanical sensorincluding the electrodes. FIG. 16 is a view showing a cross section ofFIG. 18 taken along the line A—A. In the drawings, an IC circuit, wiresand the like formed in an SOI circuit are omitted and externalcontacting aluminum electrodes alone are shown as an electrode forcontacting a capacitance, an electrode for oscillating the weight andthe like in the sensor. In other words, all electrode contactingportions are formed on the major surface of the monocrystalline siliconsubstrate 101.

As shown in FIG. 16, the monocrystalline silicon substrate 101 is joinedto a monocrystalline silicon substrate 110 through an SiO₂ film 111. Inthis monocrystalline silicon substrate 101, the beam structure describedearlier is formed.

In FIGS. 16 and 18, in a surface of the weight 139 of the cantilever102, a movable electrode 112 is formed. The movable electrode 112includes the three projections 103, 104 and 105 of the weight 139. Inaddition, two electrodes 113 and 114 are formed below the weight 139.The excitation electrode 114 receives an alternating current electricpower and excites the weight 139 by the static electricity. In short,the movable electrode 112 and the excitation electrode 114 formexcitation electrodes.

The sense electrode 113 detects excitation of the weight 139, based onan output signal which is generated in response to excitation of theweight 139, and feedback control is performed to thereby achievepredetermined excitation of the weight 139. That is, the movableelectrode 112 and the sense electrode 113 form electrodes for excitationfeedback.

As shown in FIG. 18, on both sides of the projection 103 of thecantilever 102, fixed electrodes 133 and 134 (projection 106) are formedwhile on both sides of the projection 104, fixed electrodes 135(projection 107) and 136 (projection 108) are formed. Further, on bothsides of the projection 105, fixed electrodes 137 (projection 109) and138 are formed. In other words, the projection 103 (movable electrode112) and the fixed electrodes 133 and 134 form electrodes while theprojection 104 (movable electrode 112) and the fixed electrodes 135 and136 form electrodes. In addition, the projection 105 (movable electrode112) and the fixed electrodes 137 and 138 form faced electrodes.

FIGS. 19 to 23 show manufacturing steps. In the following, themanufacturing steps will be described.

As shown in FIG. 19, an n type (100) monocrystalline silicon substrate101 of 1 to 20 Ω·cm is prepared, and a recess portion 115 is etched in amajor surface of the monocrystalline silicon substrate 101 by dryetching or wet etching to a predetermined depth, e.g., 0.1 to 5 μm. AnSiO₂ film is formed on the major surface of the monocrystalline siliconsubstrate 101 and patterned by a photolithographic method. Followingthis, in the major surface of the monocrystalline silicon substrate 101including the bottom portion of the recess portion 115, a trench 116 ofa depth of about 0.1 to 30 μm is formed by dry etching or other suitabletechnique.

In this embodiment, a groove is formed by the recess portion 115 and thetrench 116.

On the major surface of the monocrystalline silicon substrate 101including an inner wall of the trench 116, an n⁺ type diffusion layer117 is formed which will be then covered with an SiO₂ film 118 bythermal oxidization.

Following this, as shown in FIG. 20, a polysilicon film 119 is buried inthe recess portion 115 and the trench 116 by an LPCVD method.

The surface of the polysilicon film 119 is then polished using the SiO₂film 118 as a stopper to smooth the surface of the polysilicon film 119.At this stage, the surfaces of the polysilicon film 119 and the SiO₂film 118 are preferably smoothed.

Then, in the surfaces, an SiO₂ film 120 is formed to a thickness ofabout 0.3 to 2 μm by a CVD method or other suitable method, and a bottomcontact 121 is formed at a predetermined location for electricalconnection with the n⁺ type diffusion layer 117.

Further, an n⁺ polysilicon 122 doped with As and P (phosphorus) isformed to a thickness of 0.2 to 1 μm which will serve as an electrodepattern and a shield layer.

Next, a BGSP film 123 which serves as an insulation film, for instance,is formed to a thickness of 0.2 to 1 μm in the surface. The surface ofthe BGSP film 123 is then polished and flattened.

On the other hand, as shown in FIG. 21, a silicon substrate 110 isprepared and an SiO₂ film 111 is grown into a thickness 0.2 to 1 μm in asurface of the silicon substrate 110 by thermal oxidization.

Following this, as shown in FIG. 22, the silicon substrates 101 and 110are joined to each other through the SiO₂ film 111 within N₂ at atemperature of 1000° C., for instance. A back surface of themonocrystalline silicon substrate 101 is then selectively polished usingthe SiO₂ film 118 as a stopper. As a result, the polysilicon 119 and anisolated region of the silicon substrate 101 are exposed to the surface.

An IC board and other devices (not shown) are them formed in the regionof the monocrystalline silicon substrate 101 by a known method, and analuminum wire, a passivation film and a pad window (these elements arenot shown) are formed as well.

Next, as shown in FIG. 23, the SiO₂ film 118 is removed at apredetermined region, and the polysilicon film 119 is removed at apredetermined region using an etching hole 124 which is shown in FIG.18. An etching solution may be TMAH (tetramethylammoniumhidroxide), forexample. As a result of etching, a movable electrode (beam portion) isformed.

In the semiconductor mechanical sensor fabricated in this manner, thethin monocrystalline silicon substrate 101 is joined onto themonocrystalline silicon substrate 110 through the SiO₂ film 111, and inthe monocrystalline silicon substrate 101, the cantilever 102 which hasthe weight 139 is formed at the tip. Further, in one surface of theweight 139 (the bottom surface in FIG. 16), the n⁺ type diffusion layer117 is formed with the bottom surface of the monocrystalline siliconsubstrate 101 facing the surface of the weight, and the n⁺ typepolysilicon 122 (excitation electrode 114) is formed so that the n⁺ typediffusion layer 117 and the n⁺ type polysilicon 122 form an excitationelectrode. By applying an alternating current electric power to thisexcitation electrode, static electricity is created which excites theweight 139. In addition, in the axial direction which is perpendicularto the direction of the excitation of the weight 139, the n⁺ typediffusion layer 117 is formed in one surface of the weight 139 while then⁺ type diffusion layer 117 is formed in a wall surface of themonocrystalline silicon substrate 101 facing the surface of the weight139 so that the n⁺ type diffusion layer 117 of the weight 139 side andthe n⁺ type diffusion layer 117 on the side of the wall surface of themonocrystalline silicon substrate 101 form a detecting electrode fordetecting a change in a physical quantity. The physical quantity changedetecting electrode detects a change in the electrical capacitance andhence a change in a physical quantity which acts in the same directionsuch as a yaw rate.

That is, an alternating current electric power is applied to theexcitation electrode (i.e., the n⁺ type diffusion layer 117 and the n⁺type polysilicon 122) to create static electricity and the weight isexcited by the static electricity. Under this condition, the yaw ratedetecting electrode (i.e., the n⁺ type diffusion layer 117 of the weight139 side and the n⁺ type diffusion layer 117 on the side of the wallsurface of the monocrystalline silicon substrate 101), for example,detects a change in an electrical capacitance in the axial directionwhich is perpendicular to the direction of the excitation of the weight139, whereby a change in a physical quantity which acts in the samedirection, such as a yaw rate, is detected.

Thus, in this embodiment, the recess portion 115 and the trench 116 areformed as a groove of a predetermined depth in the major surface of themonocrystalline silicon substrate 101 to thereby form the cantilever 102which has the weight 139 (first step). In inner walls of the recessportion 115 and the trench 116 which surround a substrate surface regionwhich serves as the weight 139 and the weight 139, a pair of electrodesare formed facing each other on the opposite sides of the trench 116 inthe direction of the surface of the substrate (a left-to-right directionin FIG. 19), namely, the n⁺ type diffusion layer 117. At the same time,in a substrate surface region which will serve as the weight 139, in thedirection which is perpendicular to the direction of the surface of thesubstrate (up-to-down direction of FIG. 20; the direction of thethickness of the silicon substrate 101), the n⁺ type diffusion layer 117(first electrode) is formed (second step). Next, the recess portion 115and the trench 116 are filled with a filling material, i.e., thepolysilicon film 119, and the n⁺ type polysilicon 122 (electrode) isformed on the opposite side of the polysilicon film 119 so as to facethe n⁺ type diffusion layer 117 (first electrode), followed by smoothingof the major surface of the monocrystalline silicon substrate 101 (thirdstep). The major surface of the monocrystalline silicon substrate 101and the silicon substrate 110 are then joined to each other (fourthstep). Thereafter, the back surface side of the monocrystalline siliconsubstrate 101 is then polished by a predetermined amount to thereby makethe monocrystalline silicon substrate 101 thin (fifth step). Thepolysilicon film 119 is then etched from the back surface side of themonocrystalline silicon substrate 101, whereby the cantilever 102 whichhas the weight 139 is formed (sixth step).

As a result, the semiconductor mechanical sensor comprises the thinmonocrystalline silicon substrate 101 which is joined onto themonocrystalline silicon substrate 110 through the SiO₂ film 111(insulation film), the cantilever 102 which is formed in themonocrystalline silicon substrate 101 and which has the weight 139, themovable electrode 112 which is formed in one surface of the weight 139and a wall surface which corresponds to the same, the excitationelectrode 114 (first electrode), the movable electrode 112 of the weight139, the projections 103 to 105 which are formed one surface of theweight 139 and a wall surface which corresponds to the same in the axialdirection which is perpendicular to the excitation electrode 114, andthe fixed electrodes 133 to 138 (second electrode).

Either one of the electrodes, namely, the movable electrode 112 or theexcitation electrode 114 is formed parallel to the major surface of themonocrystalline silicon substrate 101.

Further, all electrode contacting portions are formed on the samesurface of the thin monocrystalline silicon substrate 101.

Thus, the semiconductor mechanical sensor comprises the thinmonocrystalline silicon substrate 101 joined to the monocrystallinesilicon substrate 110 through the SiO₂ film 111, the cantilever 102which is formed in the monocrystalline silicon substrate 101 and whichhas the weight 139 at the tip, the excitation electrode which is formedin one surface of the weight 139 and a wall surface of themonocrystalline silicon substrate 110 facing the weight, the excitationelectrode creating static electricity and exciting the weight when analternating current electric power is applied thereto, and the detectingelectrode which is formed in one surface of the weight 139 and a wallsurface of the monocrystalline silicon substrate 110 facing the weightin the axial direction which is perpendicular to the direction ofexcitation of the weight 139, the detecting electrode detecting a changein an electrical capacitance and hence a change in a physical quantitywhich acts in the same direction.

In this manner, processes are performed stably and a device which isstable and accurate is manufactured without contamination by using asurface micro machining technique, without performing a thermaltreatment and a photolithographic process during a wafer formingprocess, especially during fabrication of an IC circuit, in a conditionwhere a wafer recess portion, a through hole and the like have beenalready formed.

Although the foregoing has described the present embodiment in relationto the case where the excitation electrode and the sense electrode areburied in the substrate, the sense electrode may be omitted to reducecost, in which case, the silicon substrate as it is may be used as theexcitation electrode, unlike the structure described above.

In addition, although the electrodes which are formed parallel to thewafer surface are used as the sense electrode and the excitationelectrode and the electrodes which are disposed in the verticaldirection are used as the fixed electrodes for detecting the Corioliseffect, in the present embodiment, the opposite is also possible. Thatis, one of the fixed electrodes which are disposed in the verticaldirection in the silicon substrate 101 may be used as the excitationelectrode, and the other one of the fixed electrodes may be used as thesense electrode for performing feedback, while the electrodes which areformed parallel to the wafer surface may be used as electrodes fordetecting the Coriolis effect.

Further, as the polysilicon film 119 for filling the recess portion 115and the trench 116 (i.e., a polycrystalline silicon film), an amorphoussilicon film or a silicon film in which a polycrystalline portion and anamorphous portion are mixed may be used.

Next, still another example of the present invention will be describedwith reference to FIGS. 24 to 30.

This example is intended to further increase output as compared with thepreceding example and to prevent destruction of the beam by excessiveshock and the like.

FIGS. 24 to 30 show steps for manufacturing the sensor. In thefollowing, the manufacturing steps will be described.

In the example of FIG. 19, as shown in FIG. 24, an Si₃N₄ film 125 havinga thickness of 200 to 2000 Å is formed by the LPCVD method afterformation of the SiO₂ film 118. In this example, the thickness of theSi₃N₄ film 125 is 500Å.

In processes similar to those of the above example, polishing andflattening of the surface as shown in FIG. 22 in relation to the aboveexample are performed.

Following this, a resist 126 of FIG. 24 is patterned to a predeterminedpattern by a photolithographic technique, and a region which will serveas the sense part of the monocrystalline silicon substrate 101 isremoved by dry etching or other suitable method as shown in FIG. 25.

Next, using the resist 126 as a mask, the SiO₂ film 118 is removed bywet etching, for example, which primarily uses hydrofluoric acid as anetchant, followed by removal of the resist 126.

In the following, for clarity of explanation, an enlarged view of aportion of the sensor part B of FIG. 25 will be referred to.

FIG. 26 shows the enlarged portion.

As shown in FIG. 27, using the Si₃N₄ film 125 as a mask, an SiO₂ film127 is grown to a thickness of 500 to 10000 Å by thermal oxidization. Inthis embodiment, the thickness of the SiO₂ film 127 is 1000Å.

Next, as shown in FIG. 28, the Si₃N₄ film 125 used as a mask duringthermal oxidization is removed by plasma etching or etching using heatedphosphoric acid. A polysilicon 128 is then grown by the LPCVD method orother suitable method, on the surface. The surface of the polysilicon128 is then selectively polished and removed using the SiO₂ film 127 asa stopper.

Further, the surface is treated with a TMAH(tetramethylammoniumhidroxide) solution. At this stage, in a peripheralportion, an IC circuit and the like are formed (not shown).

Thereafter, as shown in FIG. 29, an Si₃N₄ film 129 having a thickness of500 to 2000 Å is formed on the surface, and an n⁺ type polysilicon layer130 is formed which will serve as a stopper against excessive amplitudesof the electrode layer and the sensor. Following this, a BPSG film 131is formed as a surface protection film. This film may be formed by anSi₃N₄ film or the like. A window portion 132 is then formed.

Then, as shown in FIG. 30, the polysilicon 119 and the polysilicon 128are etched through the window portion 132 with the TMAH solution.

In this manner, a sensor which comprises a movable portion (cantilever)which is entirely surrounded by an electrode and a stopper is obtained.In such a structure, when the weight portion is excited in a directionwhich is perpendicular to the substrate, as shown in FIG. 30, since a>band b is within the range of a, there will be almost no capacitancechange created during detection of a yaw rate due to excitation. Therelation a>b is attainable in the first embodiment as well.

FIG. 31 is a view which clearly shows more detail of the overallstructure.

As described above, in the present example, since the stopper member 130is disposed above the cantilever 102, output is further increased, ascompared with the above example, and destruction of the cantilever byexcessive shock and the like is prevented.

That is, in the present example, in the first step, a groove of apredetermined depth is formed in the major surface of themonocrystalline silicon substrate to thereby form the beam which has theweight. In the second step, a pair of electrodes are formed which facedeach other on the opposite sides of the groove in a substrate surfaceregion and an inner wall of the groove which surrounds the weight in thedirection of the surface of the substrate, while the first electrode isformed in a substrate surface region which will serve as the weight in adirection which is perpendicular to the surface of the substrate. In thethird step, the groove is filled with a filling material and anelectrode which faces the first electrode through the filling materialis formed, and the major surface of the monocrystalline siliconsubstrate is smoothed. Next, in the fourth step, the major surface ofthe monocrystalline silicon substrate and the silicon substrate arejoined to each other. In the fifth step, the back surface side of themonocrystalline silicon substrate 101 is polished by a predeterminedamount to thereby make the monocrystalline silicon substrate thin.Lastly, in the sixth step, the filling material is etched from the backsurface side of the monocrystalline silicon substrate, whereby the beamwhich has the weight is formed. As a result, the semiconductormechanical sensor according to the present invention is completed.

It is to be noted that the present invention is not limited to theembodiments described above. Rather, two pairs of the sensor units maybe arranged in directions perpendicular to each other in order to detectyaw rates in the two axial directions. Further, the present invention isnot limited to a cantilever. The present invention is also not limitedto detection of a yaw rate. For instance, the excitation electrode ofthe embodiments above may be replaced with an electrode which detects acapacitance of displacement in an up-to-down direction so that thepresent invention is applied to a mechanical sensor which is capable ofdetecting displacements in two directions.

As heretofore described in detail, the present invention creates effectsby which a yaw rate sensor of the beam excitation type capacitydetection method and a method of manufacturing the same are obtained,and a semiconductor mechanical sensor which can detect movement in twoor three directions and a method of manufacturing the same are obtained.

What is claimed is:
 1. A semiconductor mechanical sensor comprising: asemiconductor substrate; a beam structure extending in spaced relationover said semiconductor substrate; a weight connected to said beamstructure and including a first mechanical force detect electrode, saidweight being movable along a predetermined direction; a secondmechanical force detect electrode facing said first mechanical forcedetect electrode of said weight; an oscillation member for oscillatingsaid weight; and a sense electrode detecting oscillation of said weightfor feedback control of oscillation of said weight; wherein movement ofsaid weight produces a change in capacitance between said firstmechanical force detect electrode and said second mechanical forcedetect electrode to enable said sensor to detect mechanical forcesacting thereon.
 2. A semiconductor mechanical sensor in accordance withclaim 1, wherein said oscillation member includes a third electrode anda fourth electrode, said third electrode being formed on said weight andsaid fourth electrode facing said third electrode.
 3. A semiconductormechanical sensor in accordance with claim 1, which is adapted to beused as a yaw rate sensor.
 4. A semiconductor mechanical sensor inaccordance with claim 2, wherein said sense electrode and said fourthelectrode are formed on the same plane.
 5. A semiconductor mechanicalsensor in accordance with claim 1, wherein said oscillation memberoscillates said weight in a direction generally perpendicular to saidpredetermined direction.